Washington (CNN) — An envelope that tested positive for the deadly poison ricin was intercepted Tuesday afternoon at the U.S. Capitol’s off-site mail facility in Washington, congressional and law enforcement sources tell CNN.

After the envelope tested positive in a first routine test, it was retested two more times, each time coming up positive, the law enforcement source said. The package was then sent to a Maryland lab for further testing.

The envelope was addressed to a U.S. senator, though the sources would not say which one.

Ricin is a highly toxic substance derived from castor beans. As little as 500 micrograms — an amount the size of the head of a pin — can kill an adult. There is no specific test for exposure and no antidote once exposed.

According to Politico, the Senator in question is Mississippi Republican Roger Wicker.

Connection to the Boston attack? On Fox News a while ago, Charles Krauthammer said there is no connection it would be a “hell of a coincidence.” However, it could be an isolated incident:

McCaskill said the letter came from an individual who frequently writes to lawmakers. She wouldn’t identify the person but confirmed officials had identified someone.